Scott Darling, a state biologist, inspects a bat at the Greely Talc Mine in Stockbridge for signs of white-nose syndrome in July 2011.

CHESTER — Even though the tricolored bat has been added to the state’s endangered species list, the state’s bat population still struggles as white-nose syndrome continues to takes a toll on Vermont’s most common bat species, the little brown bat.

Brown bats are in winter caves, but many of them are succumbing to weak immune systems caused by the syndrome, which is transmitted by a white fungus.

The disease has stricken common bats, killing the majority of them in hibernation caves over the past four years, Alyssa Bennett, a state wildlife technician, recently told the Chester Conservation Society.

While Vermont caves have been adversely affected, white-nose syndrome has been ravaging bat populations across the East Coast and in other central states as well.

“It is in winter when (bats) are in that reduced state, when they cannot fight (the fungus) off very well,” she said.

While hibernating, bats’ breathing, heartbeat and temperature are reduced and their immune defenses are mostly shut off, Bennett said. Once the fungus gets on their bodies, usually from another bat, they wake up — which she said upsets the balance of electrolytes in their systems and depletes their fat reserves

Coming out of hibernation too early, in their weakened state, the bats freeze.

The white fungus, geomyces destructans, is visible on the bats’ snouts and wings in caves and ravages their body tissues.

For several years after the fungus was first found in New York bats in 2006, biologists couldn’t say for sure if the white substance was the cause of death in millions of bats nationwide.

“We have confirmed the fungus is the cause,” Bennett said.

The tricolored bat, also a cave dweller, was the third bat species to be placed on Vermont’s endangered list last fall.

One of the smallest bats found in the Northeast, the tricolored — with red, brown and yellow colors — is also one of the first to go to caves in autumn. In summer, this bat prefers the borders of woods, sleeping in upper parts of trees, often near farmland.

According to Bennett, a key reason for protecting bats like this one is the vast benefit they bring in killing insects. They save the agriculture industry $3.7 billion or more each year nationwide, she said.

The little brown bat, one of the most common American species, and the long-eared bat — which has been hardest hit in the Northeast — were the first two species put on Vermont’s endangered list 1˝ years ago, Bennett said. It’s now illegal to harm protected bats without a permit.

The disease hasn’t killed nearly as many larger bats, such as the big brown bat, a more solitary species with larger nose and teeth which can spend winters in buildings, Bennett said. Bigger bats may have a stronger immune system to fight off the fungus, she said.

White-nose syndrome has consistently taken its worst toll on smaller bats like the little brown bat in winter at caves like the Aeolus caves in East Dorset which are the region’s largest winter quarters for bats. Ninety percent of the hundreds of thousands of bats once living there have died.

White-nose syndrome has spread south in bat colonies in caves in the Appalachian Mountains, as far south as Tennessee and Alabama. The disease has been found in a total of 19 states, plus four Canadian provinces.

“This was the predicted path this disease would spread,” Bennett said. “(We) didn’t think it would spread that far that fast.”

In Vermont, Bennett has tallied numbers of surviving little brown bats in summer colonies, verifying that they continue to produce “pups” or offspring. She said she responds when people call her with reports of bats in buildings, in an effort to help protect the population.

Researchers have found the fungus in soil around bat caves. The fungus seems to prefer the same conditions favored by bats, researchers report. While bats usually catch the fungus from another bat, they can get it from the cave environment also, Bennett said.

Wildlife officials have been guarded about the locations of surviving bat populations. For the safety of the bats, specific locations are federally protected.